Refactor: Rename and Update Model Type Checking in User Action Dropdown

What does this MR do and why?

Fixes #387377 (closed)

Changes Made

1. File Name Change:
   • Renamed leave_group_dropdown_item.vue to leave_dropdown_item.vue to maintain consistency with the file naming convention.
2. Model Type Checking Update:
   • Updated the showDropdown() method in user_action_dropdown.vue (line 81) to check the model type.
   • Now, it uses “Leave Group” for group members and “Leave Project” for project members.

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